Large volumes of machining and metalworking fluids (MWFs) are used in manufacturing industries for cooling and lubrication of a variety of substrates—metals, plastics, ceramics—and tools during machining. Heat removal from the chip surface and chip-tool interface involves both macroscopic and microscopic mechanisms. Macroscopic heat elimination involves a fluid contacting chip, workpiece, and tool to remove machining heat and microscopic heat management involves reactive fluid constituents entering and reacting with chip-tool interface to reduce friction.
Increasing the velocity of a metalworking fluid is a conventional means for improving heat removal. Boundary layer velocities increase, turbulence increases and deeper cutting zone penetration can be achieved through increased fluid velocity (pressure), all of which improves heat removal. In the prior art a variety of methods are taught for forming and delivering high pressure and high velocity metalworking fluids. For example in an article entitled “Using High-Pressure Fluids”, R. Aronson, Manufacturing Engineering, June 2004, Vol. 132, No. 6, Aronson discusses the importance of matching the metalworking fluid velocity to that of the cutting tool to allow the fluid to enter the cut zone to remove heat and chips. A number of challenges are associated with increased pressure. These include a need for expensive high pressure pumps and machining tool seal modifications to handle increased pressure.
However besides fluid pressure, a number of other factors can be considered to improve boundary layer fluid velocity. Metalworking fluid properties such as viscosity, surface tension, molecular size, and reactivity predominate at the cutting surfaces and within capillary interfaces. These physicochemical properties can be optimized to allow the metalworking fluid to penetrate the cutting zone more efficiently and to flow along surfaces at much higher velocities. For example, lowering viscosity and surface tension of the metalworking fluid is a way of improving boundary layer velocity to enhance heat extraction.
A conventional method for lowering viscosity and surface tension is to use base stock fluids having extremely low viscosities and surface tension. For example, high pressure liquid or supercritical carbon dioxide (aka dense fluids) can be used as a bulk metalworking fluid (solvent), into which lubricant additives (solutes) are added, to provide the necessary reactive boundary layer constituents. However, like the high pressure metalworking fluid discussed above, these newer approaches are particularly expensive and complicated requiring specially designed high pressure plumbing in machining tools. For example, the fluid seals of most machining tools cannot tolerate the pressures and in particular the sealing dynamics (zero surface tension and very low viscosity) required to maintain and transport a dense fluid through the machining conduits.
A newer technique developed by the present inventor for improving penetration utilizes high velocity solid phase carbon dioxide composite coolant-lubricant sprays. Composite CO2 sprays resolve the high pressure limitations of dense fluids by delivering the beneficial chemistry of a dense fluid within a solid phase packet which is delivered in relatively low-pressure compressed air. A drawback of this approach is that a composite spray cannot be maintained as a solid-gas-liquid mixture through long distances through tortuous plumbing schemes found in many machining tools. A patent issued to the present inventor related to same, U.S. Pat. No. 8,048,830, teaches the use of a carbonated cutting fluid for machining applications. In the '830 invention, the present inventor utilizes a pressure pot or vessel to saturate a cutting fluid with carbon dioxide gas via high pressure compression into a machining fluid. A major drawback with this approach is that the use of pressure and a pressure vessel is both expensive and dangerous to workers. Another drawback is the cutting fluid must be delivered to the cutting zone under pressure using a suitable spray nozzle as taught in '830 or under pressure through a machining spindle to maintain gas saturation. This complicates delivery and adds considerable cost. Moreover, the invention of '830 produces excessive (or supersaturation) frothing of cutting fluid formulations. The formation of excessive foamy fluids is problematic for both machining processes and equipment (i.e., pump cavitation problems, sump overflow issues).
The present inventor has developed a much more efficient and more stable approach. The present invention utilizes non-aqueous cutting fluids (or mixtures and additives with same) having very low freeze points, and which exhibit minimal frothing, and which allow for the saturation with carbon dioxide under ambient pressure and at a lower ambient temperature. Using this approach both super-saturation and a large temperature gradient in the bulk fluid can be achieved, both of which are beneficial for cutting operations.
Following machining operations, cutting fluids which have accumulated on cut substrates and fixtures must be removed prior to the next manufacturing step. Conventionally, most cut substrates and fixtures are cleaned using aqueous, semi-aqueous or organic solvent-based methods. These conventional cleaning methods produce waste streams including unusable recovered cutting fluids containing solvent or water residues, sludge, and wastewater. These processes also consume significant amounts of energy for drying, pumping, treating and recovering cleaning fluids. New hybridized processes are needed that integrate both machining and cleaning fluid operations since both processes are inextricably linked.
Today, the predominant mode of cooling and lubricating a substrate during machining involves temperature-controlled flooded applications of various cutting fluids. The predominant mode of cleaning a cut substrate involves aqueous cleaning fluids, deionized water rinsing, and hot air drying processes. Both cutting fluids and cleaning fluids are necessary in most cases, however conventional approaches produce vast amounts of waste by-products and consume huge amounts of natural resources and energy. There exists a need to improve the performance of these fluids and operations.
As such the present invention provides a means for boosting cooling and lubrication performance of conventional cutting fluids and delivery systems, and provides a means for cleaning cut substrates and recovering residual cutting fluids for use within the machining operation. Cleaner machining processes produce higher quality cut and cleaned substrates with higher productivity using less energy and do not produce waste cutting fluids and cleaning fluids that must be treated or hauled away.